The invention relates to a dryer screen comprising in cross section flat machine direction threads i.e. warp threads, and in cross section substantially round strongly shrinking cross direction threads i.e. weft threads, said threads forming a single-layer wire cloth, which is subjected to heat treatment after weaving so that the cross direction shrinkage of the wire during heat treatment has shifted the warp threads closer to one another.
Dryer screens are used on a dryer section of a paper machine. The dryer screens allow the paper web to be dried to be guided through the dryer section. The fabric of the dryer screen is formed of threads enduring high temperatures and humidity using appropriate bindings. The dryer screen should have a particular permeability in order to make the drying of the web more efficient. Then again a high permeability may cause problems particularly in high-speed (approximately 2000 m/min) machines. Uncontrollable air-flows reduce the runability of the wire. As for modern paper machines more and more attention is paid to the aerodynamic properties of the dryer screen. Particularly the air carried by the wire causes runability problems, and therefore wires with a surface that is as smooth as possible have been developed. The idea is to make the wire as thin as possible in order to avoid runability problems caused by speed differences between the wire and the web. Furthermore, the dryer screen should be such that the marking of the web to be dried remains insignificant. This is why attempts are made to provide dryer screens with an even surface structure on the side of the web in order for the web surface to remain as smooth as possible. The web should also be appropriately dried using very little energy and also as rapidly as possible, so that the length of the paper machine remains reasonable. Consequently the contact area of the wire and the number of contact points become important.
It is known in the art that the use of flat threads increases the size of the contact area on the paper side of the dryer screen. Widening the width of flat threads also increases the contact area, but simultaneously reduces the number of contact points per surface area, thus weakening the drying capacity of the dryer screen. This is due to the fact that a reduction of contact points results in a reduction of the number of points pressing the paper web.
FI publication 96885 discloses a dryer screen in which flat machine direction threads pass over at least three, even up to nine, cross threads on the paper side. Furthermore the flat machine direction threads bound together are placed abreast by shrinking the cross threads so that the sides of the machine direction threads are grouped to face one another and thus to form a broader warp thread. A large contact area is provided on the paper side of the wire by passing the flat warp threads set to face one another at the sides for a long distance over the cross threads on the paper side. However, in such a structure the number of contact points on the paper side is small and consequently the drying properties of the wire are inadequate.
It is an object of the invention to provide an improved dryer screen having a plurality of contact points in addition to a large contact area on the paper side as well as a very smooth paper side surface.
The dryer screen of the invention is characterized in that each warp thread in the wire cloth passes above two weft threads on the paper side of the cloth, below one weft thread on the machine side of the cloth and correspondingly onwards, that the adjacent warp threads show a shift of one weft thread in the machine direction, each warp thread passing one weft thread in relation to the previous warp thread from a different point to the paper side of the cloth and correspondingly to the machine side, that after weaving the paper side surface of the cloth shows empty spaces at the points where the warp threads pass below the weft thread, and that each warp thread is as a result of the strong shrinkage of the weft threads overlapping in relation to the adjacent warp threads at said empty spaces on the paper surface.
The essential idea of the invention is that flat threads are used as machine direction threads, or as warp threads. Threads that are substantially round in cross section and also strongly shrinking are used as cross direction threads, or as weft threads. These threads are used to weave a single-layer fabric, in which the warp thread passes above two weft threads, below a weft thread and continues repeating the same pattern. The other warp threads pass in a corresponding way except that the adjacent warp threads always show a phase shift of one weft thread in relation to the previous warp thread depending on whether the warp thread passes on the paper side or correspondingly on the machine side. In accordance with said phase shift empty spaces are formed on the paper side surface at the points where the warp thread passes below the weft thread. After weaving the basic fabric is subjected to heat treatment, whereby the weft threads shrink powerfully, and as a result they shift the warp threads in the cross direction towards one another, thus narrowing the entire wire. The shrinkage is dimensioned to be so strong that the warp threads overlap with the adjacent warp threads at said empty space on the paper side. Consequently the empty spaces on the paper side are partly filled in the cross direction of the wire on account of the warp threads pushed to said space from both sides.
An essential idea of a preferred embodiment of the invention is that the contact area of the paper side of the wire is 40% or more and that the number of contact points is at the same time 65/cm2 or more.
The invention provides such an advantage that the paper side of the dryer screen is very smooth and the contact area thereof is large. On account of the overlapping warp threads the cover factor of the warp threads is high and the number of contact points is significantly larger than in previous solutions. The smooth surface prevents the marking to the product. In addition the affinity, or the force keeping the web in position, is good on a smooth wire, and thereby the wire controls the course of the web also at high speeds. Another advantage of a smooth-surfaced wire is that the wire is kept clean, and can easily be cleaned in case it is dirtied. The extensive contact area and the large number of contact points simultaneously allow an appropriate heat transmission between the web and the wire.
The wire of the invention provides good running properties. This is due to the fact that the amount of air conducted by a smooth-surface single layer wire is small. Furthermore the wire is very thin, preferably 1.3 mm or less, and the wire has an asymmetric structure, which in turn reduces the difference in running speed of the wire and the web. The extensive contact area and the large number of contact points also provide a higher drying power for the wire. Preferably the level difference of the warp threads on the paper side surface of the wire is then below 0.1 mm.
Still another advantage of the invention is that the wire does not necessarily need to be further processed after weaving and heat treatment, instead it immediately provides the designed properties and it can be directly introduced. Thus, the time consuming mechanical finishing causing additional costs, such as grinding and calendaring, can be left out.
According to previous conceptions (for example U.S. Pat. No. 5,840,637) a single-layer wire is not firm enough to be used as such on a drying section of a paper machine. However, the single-layer wire of the invention is provided with the required stability as the overlapping warp threads are obtained by strongly shrinking the weft threads. In the running tests performed no problems were noted in the running ability of the dryer screen regarding stability.
The dryer screen of the invention is applicable to be used in particular in what are known as single fabric applications, which are common at least in the front end of the drying section of the new high-speed paper machines. In single fabric application the web is conducted merely under the control of a single wire, and not in the conventional way under the control of two wires. Since the single fabric application is generally at the front end of the drying section, the web arriving thereto is still very wet. The wire of the invention is therefore preferable, since a smooth and even the wire as well as an adequate web support owing to the extensive contact area and the large number of contact points intensify the drying of the wet web. The wire also efficiently prevents the marking in the single fabric application. It is commonly known in the art that a difference in running speed exists between the web and the wire in single fabric application. The thin wire of the invention having an asymmetrical structure can be used to successfully reduce said speed difference.